Is a lowball offer Onah if the seller is ignorant about the true value?

If I know the market value for a piece of real estate or a product but the seller is clueless. Can I make an extremely low ball offer or does this constitute Onah which is forbidden?

So long as the seller is in a normal state of mind then a buyer can make any offer. If the seller accepts the offer you are in luck.

If the seller is not in a normal healthy state of mind then it may be problematic.